This invention relates to an ink-jet printer which uses an ink containing fine solid particles of a pigment suspended in a carrier liquid. More particularly, the ink-jet printer is of the type utilizing electrophoresis of the pigment particles in the ink in an ink chamber of the print head for concentrating the particles in the vicinity of an ink ejection orifice provided at an end of the ink chamber.
In known ink-jet printers of the above-mentioned type, the ink chamber in the print head is provided with a first electrode to which a steady DC voltage is applied to produce an electric field in the ink chamber thereby to induce electrophoresis of the electrically charged pigment particles in the ink toward the ink ejection orifice. As the pigment particles migrate toward the orifice at a definite rate, the particles are concentrated in the vicinity of the orifice. A second electrode is disposed in the ink chamber close to the orifice. After concentrating the pigment particles in the vicinity of the orifice, a DC voltage in pulse form is applied to the second electrode to cause ejection of an agglomeration, or agglomerations, of the pigment particles together with a small amount of the carrier liquid from the orifice toward a recording surface. By repeating this process while the ink chamber is replenished with the ink, an image is printed on the recording surface.
When the pulse duration of the voltage pulse applied to the second electrode is relatively short, a single agglomeration of pigment particles is ejected by each pulse, and this agglomeration of pigment particles forms a single dot on the recording surface. When the pulse duration is relatively long, each pulse causes ejection of a few or several agglomerations of pigment particles one after another at nearly constant time intervals, and on the recording surface these agglomerations form a single dot of a relatively large size. In this case the dot size depends on the number of agglomerations ejected by each pulse.
In the operation of the ink-jet printer described above, there is a problem that by variations in the ambient temperature, the ejection of agglomerations of pigment particles becomes unstable. In the case of ejecting only one agglomeration of pigment particles to form each dot on the recording surface, there arises a possibility of a failure in surely ejecting an agglomeration by each voltage pulse applied to the second electrode. A resultant lack of dots causes degradation of the printing quality In the case of ejecting a plurality of agglomerations of pigment particles to form each dot, the number of agglomerations ejected by each voltage pulse becomes variable so that variations occur in dot size. Nonuniformity of dot size leads to degradation of the printing quality.